Four new materials are available for grape growers: Adament, Inspire Super ­(difenoconazole + cyprodinil), Unicorn, and Vivando (metrafenone). Adament and Inspire Super, along with the existing products Flint (trifloxystrobin) and ­Pristine ­(pyraclostrobin + boscalid), ­provide the added benefit of also controlling Botrytis bunch rot, when applied at proper rates. Bunch rot was a problem in many eastern Washington vineyards last year.

The accompanying fungicide tables include fungicide class information and the Fungicide Resistance Action Committee group number, or code. The FRAC code represents the mode of action of the fungicide, and such information is helpful when designing a fungicide program that conforms to FRAC resistance management guidelines. It is important to remember that if a pathogen population develops resistance to fungicides within a FRAC group, it is likely to be resistant to all ­members of that group. Resistance is more likely to develop if the pathogen is ­frequently treated with one or multiple fungicides within a given FRAC group.

Included in the table are members of the fungicide classes (or FRAC groups) known as DMI (demethylation inhibitors, Group 3), QoI (quinone outside inhibitors; previously called strobilurins, Group 11), quinolines (quinoxyfen, Group 13), sulfur (Group M2), various ­“biological” fungicides (Group 44), ­benzophenones (metrafenone, Group U8), petroleum-derived spray oils, and potassium bicarbonate. Petroleum spray oils and potassium bicarbonate are listed as “Not Classified” (NC) by FRAC. Several products are formulations or “premixes” of two different fungicide classes, modes of action, of FRAC groups. Consult product labels for appropriate rates and spray intervals. The table also lists the resistance risk, which is product-dependent. All of the aforementioned “new” products have performed well in efficacy ­trials at Washington State ­University ­Irrigated Agricultural Research and Extension Center in Prosser.

New premix trend

The availability of “premix” or combination fungicide formulations is a relatively recent trend in agriculture. The fungicide toolbox contains several of these product types: Adament (tebuconazole + trifloxystrobin), Pristine ­(pyraclostrobin + boscalid), Inspire Super (difenoconazole + cyprodinil), and Unicorn (tebuconazole + sulfur). Both active ingredients in these compounds—with the exception of Inspire Super—have activity against powdery mildew. For Inspire Super, only the tebuconazole component is active against the disease. When both modes of action have activity against the target organism, some level of resistance management is built into the products, provided that they are used rationally. The use of “premix” types of products can provide better disease control, provide disease control security if there is field resistance to one of the two active ­ingredients, and help prevent resistance if there is not.

QoI (Group 11) or QoI-containing fungicide products are part of the cherry (Abound, Adament, Cabrio, Gem, and Pristine) and grape (Abound, Adament, Flint, Pristine, and Sovran) industries’ first line of defense against powdery mildews. The resistance risk of these Group 11 fungicides (formerly known as strobilurins) is high, while the risk of other important classes (DMI and quinolines) is considered medium. The resistance risk of the contact fungicides sulfur, narrow-range petroleum oil, and potassium bicarbonate is low. We have no evidence of fungicide-resistant mildew populations on either grape or cherry powdery mildews in eastern ­Washington, but this could change rapidly given the nature of powdery mildew and the resistance history of Group 11 and Group 3 fungicides. Therefore, it is imperative that resistance ­management guidelines be followed beginning with the introduction of the group.

Resistance management ­guidelines

General resistance management guidelines include the incorporation of cultural practices that lower disease pressure. Cultural practices such as vigor management and effective pruning of cherry trees both serve to lower disease pressure and improve spray penetration. In grapes, shoot removal and positioning, leaf removal, and vigor management lower disease pressure and open up the canopy for spray penetration. The incorporation of these practices serves to lower selection pressure on pathogen populations. Always use fungicides in a protective, rather than reactive, manner: It is far easier to ­prevent powdery mildew than to cure it.

Additional resistance management guidelines include:

Limit the number of applications of individual modes of action per season, and limit sequential applications.

Avoid tank mixing or alternating fungicides with the same FRAC ­number in a spray program.

Apply medium-risk compounds such as DMI (Group 3) and quinoline compounds (Group 13) no more than three times per season and no more than twice in sequence.

Try to make only one application of any resistance-prone compound and then switch to a fungicide from a different class or FRAC group, though the cost of this approach can be expensive in eastern Washington.

Never exceed more than two QoI applications in sequence. If two sequential ­applications of a QoI fungicide are made, this “block” should be alternated with at least two applications of one or more fungicides of a different mode of action or FRAC group.

When using Group 11 compounds as a solo product (Abound, Cabrio, and Gem), make sure the number of ­applications is no greater than a third of the total number of fungicide ­applications per season.

When using tank mixes or premixes of a Group 11 fungicide with a ­fungicide of another group (such as Adament or Pristine), limit the number of Group 11 fungicide (QoI)-­containing applications to no more than half of the total number of ­fungicide applications per season. It also helps to tank-mix fungicides from ­different groups that are both effective against ­powdery mildew.

Try to include sulfur in every spray tank aimed at powdery mildew if it is permitted according to usage instructions on product labels and applying it is not detrimental to overall IPM objectives. Sulfur is a relatively inexpensive and effective companion product for mixing with medium- or high-risk compounds.

Always follow label instructions pertaining to application rates and intervals and always use a properly calibrated sprayer and sufficient spray volume to provide good coverage.

Grape spray period

The most critical period for powdery mildew control on grapevines is from the immediate prebloom to three weeks postbloom. Our most effective compounds should be ­utilized during this period. Bloom is also a critical period for the establishment of Botrytis bunch rot in the vineyard. As noted above, several of our highly ­effective grape powdery mildew fungicides/fungicide premixes (Adament, Flint, Inspire Super, and ­Pristine) provide activity against both powdery mildew and bunch rot when used according to label directions.

Trade (brand) names are provided for your reference only. No discrimination is intended, and other pesticides with the same active ingredient may be suitable. No endorsement is implied.